An air conditioner is built to cool a room to about 70 degrees, not a cold room near 35. When forced colder, the evaporator coil drops below freezing and ices over, blocking airflow and stopping cooling. Add-on controllers can help, but the unit was not designed for it, and moisture and defrost problems remain.
Many hunters and small producers build a cold room from an insulated space and a window air conditioner, sometimes with a controller that fools the unit into running colder. It can work for a while, and then the coil freezes solid.
Why the coil ices
- Air conditioner coils normally run just above freezing. At low box temperatures the coil surface drops below freezing.
- Moisture from the air freezes on the coil, and ice blocks airflow.
- With airflow blocked, the coil gets even colder and the ice grows.
- There is no defrost cycle designed for this operating range.
- The thermostat sensor on the unit reads the wrong point, so the unit does not respond to the real box temperature.
Contributing factors
| Factor | Effect |
|---|---|
| Humid Mississippi air | More moisture to freeze on the coil |
| Leaky door or box | Constant supply of new humid air |
| Dirty filter or coil | Less airflow and faster icing |
| Fan set on low | Slower airflow and colder coil |
| Heavy loading with warm meat | Extra moisture and load |
| No drain for melt water | Water refreezes in the unit |
Things that reduce the problem
- Seal the room well and keep the door closed.
- Keep the filter and coil clean.
- Use the highest fan speed.
- Do not set the unit's own thermostat lower than the manufacturer allows.
- Give the unit a rest cycle, for example by letting the coil thaw regularly.
- Use a controller designed for the purpose, and read its limitations.
What to do when it is iced over
- Turn the unit off. Let the ice melt naturally.
- Move food if needed, to another cold space.
- Do not chip the ice. Coils are thin and easily damaged.
- Dry the area and the drain.
- Restart and watch airflow and temperature for a few hours.
Honest limits
A converted air conditioner is a stopgap, not a walk-in refrigeration system. It may hold temperatures in mild weather, then fail on the hottest days, exactly when you need it. It also may not meet health rules if you sell food from the cold room. If the room matters to your business, a proper refrigeration system with a defrost cycle is the sound long-term route. See sizing and load calculation, freezing up and defrost repair and the evaporator coil.
This page is general information. Rules and guidance change, so confirm details with the agency named above or a qualified professional.
When to upgrade
If the room is used for anything important, such as customer meat or business stock, a proper walk-in refrigeration system will save trouble. Ask a technician for a quote on a purpose-built unit sized for your room, and compare it with the cost of repeated losses and repairs on the conversion.
Questions about this topic
Can a technician fix a converted cold room?
Sometimes they can improve it, but the equipment has design limits. They may recommend replacing it.
Is it safe to store meat in one?
Only if it reliably holds 41 degrees or lower. Use thermometers and check often.
Why does it work in winter and fail in summer?
Cooler weather reduces the load and the humidity, so the coil freezes less often.